Lithium secondary battery

ABSTRACT

A lithium secondary battery is disclosed. The battery comprises a positive electrode, a negative electrode, an electrolyte solution comprising an electrolyte, a separator, and a ligand. The ligand is oriented at the interface of the electrolyte solution and the positive electrode and at the interface of the electrolyte solution and the negative electrode. The ligand has a cyclic structure having a pore that has a diameter of about 1.7 angstroms or more and coordinates lithium ions more strongly than either the solvent or the electrolyte. Typical ligands are coronands (crown ethers), podanocoronands (lariat ethers), cryptands, and spherands. The battery maintains high reliability and energy density, even after storage at high temperature.

This application is a U.S. National Phase Application of PCTInternational Application PCT/JP99/06895, filed Dec. 9, 1999.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery comprisinga positive electrode having a lithium-containing composite oxide asactive material, a negative electrode having graphite capable ofreacting to insert or extract lithium as active material, a separator,and an organic electrolyte solution.

BACKGROUND ART

A lithium ion secondary battery comprising an organic electrolytesolution, a negative electrode active material of carbon material, and apositive electrode active material of lithium-containing composite oxidehas a higher voltage, a higher energy density, and an excellent lowtemperature characteristic as compared with a secondary battery ofaqueous solution. In such secondary battery, since lithium metal is notused as negative electrode, an excellent cycle stability and a highsafety are known, and it is put in practical use. Further, a lithiumsecondary battery mixing a polymer absorbing and holding an organicelectrolyte solution in the active material layer and using a separatormade of such polymer is being developed as thin and lightweight type.

Various additives have been proposed so far for improving thecharacteristics of the lithium secondary battery. In particular, it isknown that 12-crown-4-ether as crown ether (coronand) and lithium ionstrongly form a complex of 1:1, and it has been proposed to use thecrown ether as an additive for suppressing lithium dendrite (forexample, Japanese Patent Publication No. 58-12992, Japanese Laid-openPatent No. 57-141878, Japanese Laid-open Patent No. 61-8849, JapanesePatent No. 2771406, U.S. Pat. Nos. 4,132,837, and 4,520,083). As astabilizing agent in LiAs/F₆/THF system, 18-crown-6-ether has beenproposed (Proc. 34th Int. Power Sources Symp., 84, IEEE, Piscataway,N.J.).

Also in the Li/TiS₂ battery system utilizing intercalation reaction, theadditive effect of 12-crown-4-ether has been reported (J. Electrochem.Soc., 134-(1987), 2107). Further, Japanese Laid-open Patent No. 6-13110proposes to use crown ether as cosolvent or additive in the batterysystem utilizing the intercalation reaction to the negative electrodeusing graphite, and 12-crown-4-ether is disclosed as an optimum crownether for lithium cation. The content of such 12-crown-4-ether in molarnumber is required to be a molar number equal to superior to that ofelectrolyte salt, or preferably a double molar number of electrolytesalt.

Besides, as an additive for promoting intercalation reaction intographite in the PC system electrolyte solution, 12-crown-4-ether hasbeen proposed (J. Electrochem. Soc., 140 (1993), 922; J. Electrochem.Soc., 140 (1993), L101; J. Electrochem. Soc., 141 (1994), 603). Further,in the polymer battery system, similar proposals have been made(Japanese Laid-open Patent No. 61-284071, Japanese Laid-open Patent No.3-220237, U.S. Pat. Nos. 4,609,600, and 5,523,179, etc.).

As clear from these proposals, the state of solvation variessignificantly depending on the complex forming capacity and complexforming of crown ether in the organic electrolyte solution system onlithium ions. However, depending on the type of the crown ether, thecomplex may be too strong, or the effect of complex formation variesdepending on the type of the electrolyte solution solvent, and it is notput in practical use yet.

A lithium ion secondary battery of organic electrolyte solution systemcomprising a positive electrode active material having alithium-containing composite oxide, a negative electrode active materialhaving a graphite material capable of reacting to insert or extractlithium has problems such as drop of capacity in high temperaturestorage. For example, when a conventional lithium ion secondary batteryis stored for 10 days at 70° C., the battery capacity deteriorates toabout 70%.

It is hence an object of the invention to present a lithium secondarybattery of high energy density or a lithium polymer secondary batteryhaving such characteristics as small capacity drop in high temperaturestorage, favorable cycle characteristic and excellent reliability.

SUMMARY OF THE INVENTION

A lithium secondary battery of the invention comprises a positiveelectrode having an oxide containing lithium as a positive electrodeactive material, a negative electrode having a negative electrode activematerial capable of reacting chemically with the lithium, an electrolytesolution, a separator, and a ligand.

The electrolyte solution contains a solvent, and an electrolytedissolved in the solvent, and the solvent has about 20 donor number orless.

The ligand is oriented at the interface of the electrolyte solution andpositive electrode surface, and at the interface of the electrolytesolution and negative electrode surface, the ligand has a strongercoordination selecting capacity than the solvent or electrolyte againstthe lithium, and the ligand has a cyclic structure having a pore in thechemical formula, and this pore has a diameter of about 1.7 angstroms ormore.

The ligand is contained in the electrolyte solution, and the ligand iscontained in a range of 10⁻¹ to 10⁻⁴ by molar ratio to the electrolyte.

Or, the ligand is contained in the electrolyte solution, and the ligandis contained in a range of 1 micromole to 1 millimole per 1 Ah of thebattery capacity.

Preferably, the solvent contains one mixed solvent of (a) cycliccarbonate and chain carbonate, or (b) cyclic carbonate, chain carbonate,and aliphatic carboxylic acid ester, and the amount of the mixed solventcontains more than 80% of the total volume of the solvent.

Preferably, the electrolyte contains at least one selected from thegroup consisting of lithium perchlorate, lithium tetrafluoro borate,lithium hexafluoro phosphate, trifluoromethane sulfonate, andtrifluoromethane imide sulfonate.

Preferably, the ligand is at least one selected from the groupconsisting of coronand, podanocoronand, cryptand, and spherand.

Preferably, the oxide containing lithium has lithium cobalt oxide, andthe negative electrode active material has graphite.

In this composition, deterioration of battery characteristic after thesecondary battery is stored in high temperature atmosphere is remarkablyimproved, and even after storage in high temperature atmosphere, a highreliability and a high energy density are maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a cylindrical lithiumsecondary battery in an embodiment of the invention.

FIG. 2(a) is a longitudinal sectional view of a flat lithium secondarybattery in an embodiment of the invention.

FIG. 2(b) is a top view of the same battery.

FIG. 3(a) is a chemical structural diagram showing crown ether used in asecondary battery in an embodiment of the invention.

FIG. 3(b) is a chemical structural diagram showing crown ether having Liion coordinated in a secondary battery in an embodiment of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

In a lithium ion secondary battery of organic electrolyte solutionsystem comprising a positive electrode active material havinglithium-containing composite oxide, and a negative electrode activematerial having a graphite material capable of reacting to insert orextract lithium, a method of controlling the solvent structure on theelectrode interface is studied. Controlling the complex forming capacityof ligand, the concentration is optimized. Further, drawing attention tothe balance of coordination capacity of electrolyte solution solvent,supporting salt anion and ligand, they are optimized. On the basis ofresults of such experiment, it has been known that the stability in hightemperature storage can be improved. More specifically, in the lithiumsecondary battery or lithium polymer battery, a trace of lithium ionselective ligand such as coronand (crown ether) is oriented on theelectrode interface. Capacity loss in high temperature storage is mainlyconsidered to be due to self-discharge by side reaction on the positiveand negative electrode interface of electrolyte solution solvent.Accordingly, by orienting a lithium ion selective ligand on theelectrode interface, an interface state different from the electrodeinterface formed as an extension of electrolyte solution bulk is formed,and this interface state suppresses the side reaction(oxidation-reduction reaction) of the electrolyte solution solvent.

As the lithium ion selective ligand, multiple types of ligands arespecifically described in, for example, Lithium Chemistry (pp. 393-476,ed. by A. Sapse and P. von R. Schleyer, 1995, John Wiley & Sons., Inc.,Canada). A preferred ligand has a cyclic structure of coronand (crownether), podanocoronand (lariat ether), cryptand, and spherand. The crownether has a cyclic structure, for example, as shown in FIG. 3. No effectwas noted in the ligand having podant (grime). The reason is unknown,but it seems because the ligand having a chain structure is unstable,and decomposition or polymerization reaction may occur. The coordinationcapacity of the ligand to the lithium ion is also related, and if thecoordination capacity is too strong, ion conduction in the battery isimpeded, and the charging and discharging characteristic is largelylowered. The standard of coordination capacity is unclear, but one ofthe standards is the size of lithium ion or size of pore of the cyclicligand. The pore size of the cyclic ligand is estimated from the CPK(Corey-Paulimg-Kortum) model. (See Lithium Chemistry, pp. 406, 410, ed.by A. Sapse and P. von R. Schleyer, 1995, John Wiley & Sons., Inc.,Canada.) For example, the pore size of 15-crown-5-ether is 1.7angstroms, and the pore size of 18-crown-6-ether is 2.86 angstroms. Whenusing 12-crown-4-ether with pore size of 1.55 angstroms claimed to beoptimum in Japanese Laid-open Patent No. 6-13110, the discharge capacitywas substantially lowered. Therefore, as the ligand of coronand (crownether) type used in the invention, crown ether having a pore size of 1.7angstroms or more is preferred, and derivatives of 15-crown-5-ether,benzo-15-crown-5, N-methyl monoazo-15-crown-5, and others,18-crown-6-ether, its derivatives, 16-crown-5-ether, 21-crown-7-ether,and others may be used. Examples of cryptand are those having a poresize of 1.7 angstroms or more, such as 2,2,1-cryptand, and2,2,2-cryptand. Further, as the ligand, those having larger cyclicmolecules are used, such as podanocoronan (lariat ether) and spherand.

As a result of intensive studies, it is learned that the coordinationcapacity of electrolyte solution is very important in order to realizethe effects of the ligand such as mentioned above. For this purpose, thesolvent having the number of donor number of 20 or less is preferred.The number of donor number is defined, in 1,2-dichloroethane, to be thevalue of molar enthalpy (kcal/mol) in the reaction between the acceptorand the donor by selecting antimony pentachloride of 10⁻³ mol.dm⁻³ asthe standard acceptor. (See Dictionary of Science, Tokyo Kagaku Dojin,1989.)

Examples of solvent having a donor number of 20 or less includepreferably cyclic carbonates such as ethylene carbonate (16.4 donornumber) and propylene carbonate (15.1 donor number), chain carbonatessuch as diethylene carbonate (15.1 donor number), and aliphaticcarboxylic acid esters such as methyl acetate (16.5 donor number) andethyl acetate (17.1 donor number). In particular, preferred solventshaving a donor number of less than 20 are dimethyl carbonate, ethylmethyl carbonate, methyl propionate, and ethyl propionate. In thesolvent used in the lithium battery system, solvents having a donornumber of 20 or more include, for example, 1,2-dimethoxy ethane (24donor number), tetrahydro furane (20 donor number), dimethyl sulfoxide(29.8 donor number), and dimethyl formamide (26.6 donor number). Thedonor numbers are quoted from Lithium Batteries (ed. by G. Pistoia,1994, Elsevier, Amsterdam), and High-Energy Non-Aqueous Batteries (ed.by A. Cisak and L. Werblan, 1993, Ellis Horwood, England).

A preferred combination of solvents is a combination of cycliccarbonate, such as ethylene carbonate or propylene carbonate, and chaincarbonate, such as dimethyl carbonate, diethylene carbonate, or ethylcarbonate. The reason why these solvents are preferred is not known, butit is estimated because the solvation of lithium ion has a coordinatedstructure of carboxyl group (—OCO₂—) in the carbonate system solvent.That is, the cyclic carbonate and chain carbonate have a structure inwhich a carboxylic group is coordinates in lithium ions, and the lithiumions are stabilized same as in a single solvent of cyclic carbonate, andhence it is more easily stabilized than in a mixed solvent using otherchain ester. Further, the mixed solvent system containing chaincarbonate has a low boiling point or low viscosity, the effect ofenhancing the ion conductivity notably is obtained. It is also possibleto mix aliphatic ester carbonate in the solvent as required, usableexamples of aliphatic ester carbonate include methyl acetate, ethylacetate, methyl propionate, and ethyl propionate.

Using such electrolyte solution, the amount of ligand is preferred to bein a range of 10⁻¹ to 10⁻⁴ in 1 part of supporting salt by the molarratio to the supporting salt in the electrolyte solution, morepreferably in a range of 10⁻² to 10⁻³. In the electrolyte solutioncontaining cyclic carbonate and chain-carbonate as solvent (or solventfurther mixing an aliphatic carboxylic acid ester), when the content ofcyclic carbonate and chain carbonate is more than 80% of total volume ofthe solvent, the addition of such trace is preferred. When the contentof ligand is more than 10⁻¹ in 1 part of supporting salt by the molarratio to the supporting salt, adverse effects appear in the dischargecharacteristic of the battery. The cause is estimated because a complexis formed by ligand in the electrolyte solution using the solvent of theembodiment. When the content of ligand was less than 10⁻⁴ in 1 part ofsupporting salt by the molar ratio to the supporting salt, such markedeffects were not observed.

As the paired anion of supporting salt, an ion of low coordinationcapacity is preferred. It is known that an anion having a largemolecular structure is easily dissociated, but no numerical index suchas the number of donor number of the solvent is known. Therefore, fromthe anions low in coordination capacity listed in “Inorganic Complex,Chelate Complex” (ed. by Japan Society of Chemistry, ExperimentalChemistry Lecture 17, 1990, Maruzen, Tokyo), as those applicable tolithium batteries, the following anions are selected: perchlorate(ClO₄—), tetrafluoro borate (BF₄—), hexafluoro phosphate (PF₆—),trifluoromethane sulfonate (CF₃SO₃—), and trifluoromethane imidesulfonate (N(CF₃SO₂—)₂).

Methods of orienting the ligand such as coronand on the electrodeinterface include (a) a method of dissolving the coronand in a lowboiling point solvent such as diethyl ether, applying the dissolvedsolution on the electrode surface, and evaporating the low boiling pointsolvent, (b) a method of evaporating the ligand on the electrode surfacein a vacuum chamber, and (c) a method of grinding into powder in thecase of ligand high in melting point and low in dissolution, and mixingwith active material particles. In a simpler method, the ligand isdissolved and mixed in electrolyte solution, and is oriented on theelectrode surface. For example, when the coronand such as15-crown-5-ether is dissolved in an electrolyte solution ofLiPF₆/(EC+EMC) (ratio by weight 1:1), the orientation and adsorption of15-crown-5-ether on the electrode interface can be confirmed from thepeak derived from C-O-C expansion and vibration of 15-crown-5-etherappearing near 1105 cm⁻¹ when measuring the double modulationpolarization spectrum of Fourier transform infrared spectroscopy (FTIR)(for example, ed. by M. Tasumi, Basic and Practical FTIR, 1994, TokyoKagaku Dojin; Electric Chemistry and Industrial Physical Chemistry, 66(1998), 824).

Embodiments of the invention are described below.

A lithium secondary battery in an embodiment of the invention comprisesa positive electrode having a lithium-containing composite oxide asactive material, a negative electrode having graphite capable ofreacting to insert or extract lithium as active material, an organicelectrolyte solution, and a separator. The solvent of the electrolytesolution includes a mixture of cyclic carbonate and chain carbonate. Orthe solvent of the electrolyte solution includes a mixture of cycliccarbonate, chain carbonate, and aliphatic carboxylic acid ester. Thesemixtures are preferred to be contained by more than about 80% of thetotal solvent volume. A substance containing an anion of lowcoordination capacity selected from perchlorate (ClO₄—), tetrafluoroborate (BF₄—), hexafluoro phosphate (PF₆—), trifluoromethane sulfonate(CF₃SO₃—), and trifluoromethane imide sulfonate (N(CF₃SO₂—)₂), andsupporting salt are combined to obtain the electrolyte solution. Theligand is added in the battery in a range of 10⁻¹ to 10⁻⁴ in 1 part ofsupporting salt by the molar ratio to the supporting salt, and theligand is oriented on the interface of positive electrode andelectrolyte solution layer, and on the interface of the negativeelectrode and electrolyte solution layer. The ligand has a cyclicstructure having a stronger selectivity to lithium ion than solvent oranion, and has a pore size of 1.7 angstroms or more. This compositionforms an electrode interface different from the interface of theelectrode/electrolyte solution formed as an extension of electrolytesolution bulk, and it suppresses oxidation-reduction reaction occurringas a side reaction of the solvent in the electrolyte solution. As aresult, the high temperature storage property of the lithium secondarybattery is improved.

A secondary battery in other embodiment of the invention comprises apositive electrode, a negative electrode, a separator, and an organicelectrolyte solution. The positive electrode includes an active materialmixture layer containing an active material having lithium-containingcomposite oxide and a first polymer absorbing and holding the organicelectrolyte solution, and a current collector for supporting the activematerial mixture layer. The negative electrode includes an activematerial mixture layer containing an active material having graphitecapable of reacting to insert or extract lithium, and a second polymerabsorbing and holding the organic electrolyte solution, and a currentcollector for supporting the active material mixture layer. Theseparator has a third polymer for absorbing and holding the organicelectrolyte solution, and is porous. The organic electrolyte solution isabsorbed and held in the positive electrode, negative electrode, andseparator. The solvent of the electrolyte solution includes a mixture ofcyclic carbonate and chain carbonate. Or the solvent of the electrolytesolution includes a mixture of cyclic carbonate, chain carbonate, andaliphatic carboxylic acid ester. These mixtures are preferred to becontained by more than about 80% of the total solvent volume. Asubstance containing an anion of low coordination capacity selected fromperchlorate (ClO₄—), tetrafluoro borate (BF₄—), hexafluoro phosphate(PF₆—), trifluoromethane sulfonate (CF₃SO₃—), and trifluoromethane imidesulfonate (N(CF₃SO₂—)₂), and supporting salt are combined to obtain theelectrolyte solution. The ligand is added in the battery in a range of 1micromole to 1 millimole per 1 Ah of the battery capacity, and theligand is oriented on the interface of positive electrode andelectrolyte solution layer, and on the interface of the negativeelectrode and electrolyte solution layer. The ligand has a cyclicstructure having a stronger selectivity to lithium ion than solvent oranion, and has a pore size of 1.7 angstroms or more. This compositionforms an electrode interface different from the interface of theelectrode/gel polymer electrolyte formed as an extension of bulk, and itsuppresses oxidation-reduction reaction occurring as a side reaction ofthe plasticizer solvent. As a result, the high temperature storageproperty of the lithium polymer secondary battery is improved.

Referring now to the drawings, exemplary embodiments of the inventionare described below.

FIG. 1 is a longitudinal sectional view of a cylindrical lithiumsecondary battery in an embodiment of the invention. A secondary batterycomprises a battery case 1, a polyethylene insulating plate 2, apolypropylene gasket 3, a sealing plate 4 with a safety device, analuminum positive electrode lead 5, a positive electrode plate 6 usinglithium-cobalt composite oxide or the like as active material, apolyethylene separator 7, a negative electrode plate 8 using sphericalgraphite or the like as active material, and a copper negative electrodelead 9. The positive electrode 6 is a positive plate obtained byapplying positive electrode paste on an aluminum foil as positivecurrent collector, drying, rolling and cutting to specified size. Thepositive electrode paste is prepared by mixing and dispersing the activematerial, conductive agent such as carbon black, and binder such asfluorine resin, in an aqueous solution of carboxymethyl cellulose (CMC).The negative electrode 8 is a negative plate obtained by applying anegative electrode paste on a copper foil as negative current collector,drying, rolling and cutting to specified size. The negative electrodepaste is prepared by mixing and dispersing the spherical graphite asactive material in an aqueous solution of styrene butadiene rubber (SBR)as binder and CMC. An organic electrolyte solution is contained in thepositive electrode 6, separator 7, and negative electrode 8.

FIG. 2(a) is a longitudinal sectional view of a flat lithium secondarybattery in an embodiment of the invention, and FIG. 2(b) is its topview. A secondary battery comprises an aluminum laminate battery case11, a lead part fusion seal 12, an aluminum positive electrode lead 15,a positive electrode plate 16 using lithium-cobalt composite oxide orthe like as active material, a separator 17 for absorbing and holding anelectrolyte solution containing vinylidene fluoride-propylenetetrafluoride copolymer, a negative electrode plate 18 using sphericalgraphite or the like as active material, and a copper negative electrodelead 19. The positive electrode 16 is a positive plate obtained byapplying positive electrode paste on an aluminum foil as positivecurrent collector, drying, and cutting to specified size. The positiveelectrode paste includes lithium-cobalt composite oxide, carbon black asconductive agent, vinylidene fluoride-propylene tetrafluoride copolymeras a first polymer, N-methyl pyrrolidone as solvent, and dibutylphthalate as plasticizer. The polymer absorbs and holds the organicelectrolyte solution, and has an effect as binder. The negativeelectrode 18 is a negative plate obtained by applying a negativeelectrode paste on a copper core as negative current collector, drying,and cutting to specified size. The negative electrode paste includesspherical graphite as active material, vinylidene fluoride-propylenetetrafluoride copolymer as a second polymer, N-methyl pyrrolidone assolvent, and dibutyl phthalate as binder. The polymer absorbs and holdsthe organic electrolyte solution, and has an effect as binder.

The separator layer 17 is obtained by applying and drying separatorpaste. The separator paste includes vinylidene fluoride-propylenetetrafluoride copolymer as a third polymer, and silicone oxide finegranules as structural member. The positive electrode plate 16,separator layer 17, and negative electrode plate 18 are fused by heatroller, and dibutyl phthalate is eluted in diethyl ether, and a porouspolymer electrode group is obtained. This electrode group is put in thebattery case 11, and organic electrolyte solution is poured in, and gelpolymer electrolyte is formed. Finally, the aluminum laminate at theopening is heated and fused. Thus, a flat polymer battery is obtained.The secondary battery of the embodiment is shown as cylindrical batteryand flat battery, but not limited to these shape, the secondary batteryof the invention may be formed in various shapes such as square batteryand coin type battery, and the same effects as mentioned above areobtained in such constructions.

In the secondary battery of this exemplary embodiment, the positiveelectrode defines the capacity, and the discharge capacity is determinedby the utility rate of the positive electrode active material.

Specific embodiments of the invention are shown below.

EMBODIMENT 1

A lithium battery in an embodiment of the invention is a cylindricallithium secondary battery shown in FIG. 1. As the positive electrodeactive material, lithium-cobalt composite oxide (LiCoO₂) was used. Asthe negative electrode active material, spherical graphite was used. Anelectrode group having a design discharge capacity of 600 mAh wasprepared. In a mixed solvent of ethylene carbonate (EC, 16.4 donornumber) and ethyl methyl carbonate (EMC, 20 donor number or less), bydissolving lithium phosphoric acid tetrafluoride (LiPF₆) as electrolyte,an electrolyte solution was prepared. The rate of the mixed solvent isEC:EMC=5:5 as the ratio by weight. The electrolyte solution contains theelectrolyte at a concentration of 1.0 mole.dm⁻³. In this electrolytesolution, further, 15-crown-5-ether (C₁₀H₂₀O₅, pore size 1.7 angstroms,coronand, abbreviated as 15-C-5-E) was added as ligand. Theconcentration of the ligand in the electrolyte solution is 10⁻² moledm⁻³. The structural formula of 15-crown-5-ether is shown in FIG. 3. Asshown in FIG. 3(a), 15-crown-5-ether has a cyclic structure as chemicalstructure, and the diameter of the cyclic part is about 1.7 angstroms.As shown in FIG. 3(b), the lithium ion can be coordinates in this cyclicstructure.

The design discharge capacity means the capacity calculated at theutility rate of LiCoO₂ of 120 mAh/g.

EMBODIMENT 2

As the ligand, 15-crown-5-ether is added in the electrolyte solution atconcentration of 10⁻¹ mole.dm⁻³. The other composition is same as inembodiment 1.

EMBODIMENT 3

As the ligand, 15-crown-5-ether was used. This ligand is added in theelectrolyte solution at concentration of 10⁻³ mole.dm⁻³. The othercomposition is same as in embodiment 1.

EMBODIMENT 4

As the positive electrode active material, spinel type lithium-manganesecomposite oxide (LiMn₂O₄) was used. An electrode group having a designdischarge capacity of 500 mAh (calculated by supposing the positiveelectrode utility rate to be 100 mAh/g) was prepared. The designdischarge capacity of 500 mAh is the value calculated by supposing thepositive electrode utility rate to be 100 mAh/g. As the solvent of theelectrolyte solution, a mixed solvent of EC, diethyl carbonate (DEC,15.1 donor number), and ethyl acetate (EA, 17.1 donor number) was used.The composition of the mixed solvent as the ratio by weight isEC:DEC:EA=4:4:2. The concentration of the positive electrode activematerial LiPF₆ is 1.2 mole.dm⁻³. As the ligand, 18-crown-6-ether(C₁₂H₂₄O₆, pore size 2.86 angstroms) was used. This ligand is added inthe electrolyte solution at a concentration of 2×10⁻² mole.dm⁻³. Theother composition is same as in embodiment 1.

EMBODIMENT 5

As the solvent of the electrolyte solution, a mixed solvent of ethylenecarbonate (EC, 16.4 donor number), dimethyl carbonate (DMC, 20 donornumber or less), and methyl propionate (MP, 20 donor number or less) wasused. The composition of the mixed solvent as the ratio by weight isEC:DMC:MP=4:4:2. As the electrolyte, lithium phosphoric acidtetrafluoride (LiPF₆) was used, and the concentration of thiselectrolyte is 1.5 mole.dm⁻³. As the ligand, cryptand was used, that is,2,2,1-cryptand (C₁₆H₃₂N₂O₅, pore size 1.7 angstroms or more) was used,and the concentration of this ligand is 10⁻² mole.dm⁻³.

EMBODIMENT 6

A lithium battery in other embodiment of the invention is a flat lithiumsecondary battery as shown in FIG. 2(a) and FIG. 2(b). The positiveelectrode active material is lithium-cobalt composite oxide. Thenegative electrode active material is spherical graphite. The separatoris vinylidene fluoride-propylene tetrafluoride copolymer. Using thesemembers, an electrode group having a design discharge capacity of 200mAh was prepared. The design discharge capacity was calculated bysupposing the positive electrode utility rate to be 120 mAh/g. In amixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate(EMC, 20 donor number or less), by dissolving lithium phosphoric acidtetrafluoride (LiPF₆) as the electrolyte, an electrolyte solution wasprepared. The ratio of composition of the mixed solvent by weight isEC:EMC=5:5. The electrolyte solution contains the electrolyte at aconcentration of 1.5 mol.dm⁻³. Further, in the electrolyte solution,15-crown-5-ether (coronand) was added as ligand. The concentration ofthe ligand in the electrolyte solution is 5×10⁻³ mol.dm⁻³. Theelectrolyte solution containing this ligand was poured into theelectrode group. Thus, the gel polymer electrolyte battery was obtained.

Comparative Example 1

Using the electrolyte solution not containing ligand, a secondarybattery of comparative example 1 was prepared. The other composition issame as in embodiment 1.

Comparative Example 2

Using the electrolyte solution by adding 15-crown-5-ether atconcentration of 3×10⁻¹ mol.dm⁻³, a secondary battery of comparativeexample 2 was prepared. The other composition is same as in embodiment1.

Comparative Example 3

The electrolyte solution contains a mixed solvent of ethylene carbonate(EC, 16.4 donor number) and dimethoxy ethane (DME, 24 donor number), thecomposition of the mixed solvent by weight is EC:DME=5:5. The othercomposition is same as in embodiment 1.

Comparative Example 4

As the ligand, the coronand having a small pore size was used, that is,12-crown-4-ether. This 12-crown-4-ether has a smaller pore size than the15-crown-5-ether. The concentration of the ligand is 5×10⁻² mole.dm⁻³ inthe electrolyte solution. The other composition is same as in embodiment1.

Comparative Example 5

The electrolyte solution contains a mixed solvent of ethylene carbonate(EC) and ethyl methyl carbonate (EMC), the composition of the mixedsolvent by weight is EC:EMC=5:5. As the ligand, 15-crown-5-ether wasused. The concentration of the ligand is 5×10⁻⁵ mole.dm⁻³ in theelectrolyte solution. The other composition is same as in embodiment 6.Such gel polymer electrolyte battery was prepared.

Results of Measurement

In the batteries of embodiments 1 to 6 and comparative examples 1 to 5,the discharge capacity before storing in high temperature atmosphere andthe discharge capacity after storing in high temperature atmosphere weremeasured, and the specific capacity after storage at high temperaturewas comparatively studied. The method of experiment and results ofmeasurement are given below. The batteries were charged and dischargedin cycles in the charge and discharge conditions of 20° C. atmosphere,charge end voltage of 4.2 V (4.3 V in the battery of embodiment 4),discharge end voltage of 3.0 V, and 5-hour rate constant current. In thecharge and discharge cycle experiment, the charge and dischargecapacities of the batteries were measured. The batteries in chargedstate were stored for 10 days in the high temperature atmosphere of 70°C. After the storage period, the batteries were discharged in the sameconditions as before storage in high temperature atmosphere, again, inthe atmosphere of 20° C., and the discharge capacity was measured. Thus,the difference in capacity before and after storage in high temperatureatmosphere was measured, and the capacity deterioration in hightemperature atmosphere was measured. Supposing the discharge capacitybefore storage in high temperature atmosphere to be 100, the specificcapacity after high temperature storage is shown in Table 1a and Table1b.

TABLE 1a Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment5 Embodiment 6 Type of electrolyte LiPF₆ LiPF₆ LiPF₆ LiPF₆ LiPF₆ LiPF₆Concentration of electrolyte 1 1 1 1.2 1.5 1.5 (mol/dm³) Type of solventEC:EMC EC:EMC EC:EMC EC:DEC:EA EC:DMC:MP EC:EMC Mixing ratio 5:5 5:5 5:54:4:2 4:4:2 5:5 (by weight) Number of donor 20 or less 20 or less 20 orless 20 or less 20 or less 20 or less number of solvent Type of ligand15-C-5-E 15-C-5-E 15-C-5-E 18-C-6-E 2.2.1-cryptand 15-C-5-EConcentration of ligand 10⁻² 10⁻¹ 10⁻³ 1.67 × 10⁻² 6.7 × 10⁻³ 3.33 ×10⁻³ (mol/dm³) Pore size of ligand (Å) 1.7 1.7 1.7 2.86 1.7< 1.7Concentration of ligand 46 460 4.6 110 46 26 (μm/Ah) Capacity rate after86 80 82 84 85 85 storage in high temperature atmosphere (%)

TABLE 1b Comparative Comparative Comparative Comparative Comparativeexample 1 example 2 example 3 example 4 example 5 Type of electrolyteLiPF₆ LiPF₆ LiPF₆ LiPF₆ LiPF₆ Concentration of electrolyte 1 1 1 1.2 1.5(mol/dm³) Type of solvent EC:EMC EC:EMC EC:DME EC:EMC EC:EMC Mixingratio 5:5 5:5 5:5 5:5 5:5 (by weight) Number of donor 20 or less 20 orless 20< 20 or less 20 or less number of solvent Type of ligand Notcontained 15-C-5-E 15-C-5-E 12-C-4-E 15-C-5-E Concentration of ligand 03 × 10⁻¹ 10⁻² 5 × 10⁻² 3.3 × 10⁻⁵ (mol/dm³) Concentration of ligand 01.380 46 230 0.26 (μm/Ah) Pore size of ligand (Å) — 1.7 1.7 1.55 1.7Capacity rate after 73 68 62 70 70 storage in high temperatureatmosphere (%)

As clear from the table, in the secondary batteries of embodiment 1 toembodiment 6, the capacity after storage in high temperature atmospheremaintains more than about 80% of the capacity before storage. Bycontrast, in the secondary batteries of comparative example 1 tocomparative example 5, the capacity after storage in high temperatureatmosphere is decreased to about less than 80% of the capacity beforestorage. That is, the secondary batteries of embodiment 1 to embodiment6, regardless of the type of the active material of the positiveelectrode, have an excellent maintenance rate of the capacity after hightemperature storage. That is, in the secondary batteries of embodiment 1to embodiment 6, it is understood that side reaction(oxidation-reduction reaction) on the interface of organic solvent andpositive and negative electrodes such as ethylene carbonate (EC) andethyl methyl carbonate (EMC) is suppressed.

However, in the secondary battery containing too much ligand as incomparative example 2, or in the secondary battery using 1,2-dimethoxyethane copolymer having 24 donor number as in comparative example 3,when stored in high temperature atmosphere, a larger capacity drop wasobserved than the secondary battery of comparative example 1 notcontaining ligand. In the secondary battery of comparative example 4using coronand having a small pore size of 1.55 angstroms(12-crown-4-ether), before storage at high temperature, polarization indischarge was significant, and the actual capacity was lower as comparedwith the design discharge capacity. This is considered because the12-crown-4-ether is strongly coordinate in the lithium ion, and thediffusion (move) of lithium ion at the electrode interface is impeded.

As clear from the comparison between embodiment 6 and comparativeexample 5, the same effects were obtained also in the flat lithiumsecondary batter adding a polymer having a property of absorbing andholding the electrolyte solution in the positive electrode and negativeelectrode.

As clear from the description herein, paying attention to the balance ofthe solvent in the electrolyte solution, supporting salt anion andcoordination capacity of ligand, by optimizing them, the stability ofpositive electrode and negative electrode after storage in hightemperature atmosphere can be improved. As a result, the lithiumsecondary battery of excellent high temperature storage property,excellent reliability, and high energy density is obtained. Inparticular, these effects are further enhanced when the ligand is addedin the battery by a molar ratio in a range of 10⁻¹ to 10⁻⁴ to thesupporting salt, and the ligand is oriented on the interface of theelectrode surface and electrolyte solution.

In the embodiments, only LiPF₆ is used as the active material, but notlimited to this, for example, it is also possible to use lithiumperchlorate (LiClO₄), lithium tetrafluoro borate (LiBF₄), lithiumtrifluoromethane sulfonate (LiCF₃SO₃), and lithium trifluoromethaneimide sulfonate (LiN(CF₃SO₂—)₂), and same effects are obtained.

INDUSTRIAL APPLICABILITY

In a lithium secondary battery comprising a positive electrode having alithium-containing composite oxide as active material, a negativeelectrode having graphite capable of reacting to insert or desorblithium as active material, and an electrolyte, deterioration of batterycharacteristic after storage of secondary battery in high temperatureatmosphere is notably improved, and even after storage in hightemperature atmosphere, the lithium secondary battery maintaining highreliability and high energy density is obtained.

What is claimed is:
 1. A lithium secondary battery comprising: apositive electrode comprising a lithium containing oxide as a positiveelectrode active material, a negative electrode comprising graphitecapable of reacting to insert and extract lithium ions as a negativeelectrode active material, an electrolyte solution, a separator, and aligand selected from the group consisting of coronands, podanocoronands,cryptands, spherands, and mixtures thereof, wherein: said electrolytesolution comprises a solvent and an electrolyte dissolved in saidsolvent, said solvent has about a donor number of 20 or less, saidligand is oriented at an interface of said electrolyte solution and asurface of the positive electrode and at an interface of saidelectrolyte solution and a surface of the negative electrode, saidligand has a stronger coordination selecting capacity than said solventor said electrolyte for lithium ions, said ligand has a cyclic structurehaving a pore, said pore has a diameter of about 1.7 angstroms or more,and the concentration of said ligand in said battery is either (i) in arange of 1.67×10⁻² to 10⁻⁴ by molar ratio to said electrolyte, or (ii)in a range of 110 micromole to 1 millimole per 1 Ah of the capacity ofsaid battery.
 2. The lithium secondary battery of claim 1, wherein: saidelectrolyte solution comprises said ligand, and the concentration ofsaid ligand is in a range of 1.67×10⁻² to 10⁻⁴ by molar ratio to saidelectrolyte.
 3. The lithium secondary battery of claim 1, wherein: saidelectrolyte solution comprises said ligand, and the concentration ofsaid ligand is in a range of 110 micromole to 1 millimole per 1 Ah ofthe capacity of said battery.
 4. The lithium secondary battery of claim1, wherein said solvent comprises a mixed solvent of either: (a) acyclic carbonate and a chain carbonate, or (b) a cyclic carbonate, achain carbonate, and an aliphatic carboxylic acid ester.
 5. The lithiumsecondary battery of claim 4, wherein said mixed solvent comprises morethan 80% of the total volume of said solvent.
 6. The lithium secondarybattery of claim 1, wherein said electrolyte comprises at least oneanion selected from the group consisting of perchlorate, tetrafluoroborate, hexafluoro phosphate, trifluoromethane sulfonate, andtrifluoromethane imide sulfonate.
 7. The lithium secondary battery ofclaim 1, wherein said ligand is at least one ligand selected from thegroup consisting of coronands and cryptands.
 8. The lithium secondarybattery of claim 1, wherein said lithium containing oxide compriseslithium cobalt oxide.
 9. The lithium secondary battery of claim 1,wherein: said positive electrode comprises a positive current collectorand a positive electrode active material mixture layer supported on saidpositive current collector, said positive electrode active materialmixture layer comprises said positive electrode active material, and afirst polymer absorbing and holding said electrolyte solution, saidnegative electrode comprises a negative current collector and a negativeelectrode active material mixture layer supported on said negativecurrent collector, said negative electrode active material mixture layercomprises said negative electrode active material, and a second polymerabsorbing and holding said electrolyte solution, said separator has athird polymer absorbing and holding said electrolyte solution, and theconcentration of said ligand is in a range of 110 micromole to 1millimole per 1 Ah of the battery capacity.
 10. The lithium secondarybattery of claim 1, wherein said ligand is selected from the groupconsisting of 15-crown-5-ether, 18-crown-6-ether, and 2,2,1-cryptand.11. A lithium secondary battery comprising: a positive electrodecomprising a lithium-containing composite oxide as a positive electrodeactive material, a negative electrode comprising graphite capable ofreacting to insert and extract lithium ions as a negative electrodeactive material, an electrolyte solution comprising an organic solventand a supporting salt dissolved in said organic solvent, a separatordisposed between said positive electrode and negative electrode, and aligand selected from the group consisting of coronands, podanocoronands,cryptands, spherands, and mixtures thereof, wherein: said organicsolvent comprises a mixed solvent of either (a) a cyclic carbonate and achain carbonate, or (b) a cyclic carbonate, a chain carbonate, and analiphatic carboxylic acid ester, said mixed solvent comprises more than80% of the total volume of said organic solvent, said supporting saltcomprises a substance comprising at least one anion selected from thegroup consisting of perchlorate, tetrafluoro borate, hexafluorophosphate, trifluoromethane sulfonate, and trifluoromethane imidesulfonate, said ligand has a stronger coordination selecting capacitythan said organic solvent or said anion for lithium ions, said ligandhas a cyclic structure having a pore, said pore has a diameter of about1.7 angstroms or more, the concentration of said ligand in the batteryis in a range of 1.67×10⁻² to 10⁻⁴ by molar ratio in 1 part ofsupporting salt by molar ratio to said supporting salt, and said ligandis oriented at the interface of said positive electrode and saidelectrolyte solution, and at the interface of said negative electrodeand said electrolyte solution.
 12. The lithium secondary battery ofclaim 11, wherein said ligand is at least one ligand selected from thegroup consisting of coronands and cryptands.
 13. The lithium secondarybattery of claim 11, wherein said ligand is selected from the groupconsisting of 15-crown-5-ether, 18-crown-6-ether, and 2,2,1-cryptand.14. A lithium secondary battery comprising: (a) an organic electrolytesolution comprising an organic solvent and a supporting salt dissolvedin said organic solvent, (b) a positive electrode comprising an activematerial mixture layer comprising an active material comprising alithium-containing composite oxide and a first polymer absorbing andholding said organic electrolyte solution, and a positive currentcollector supporting said active material mixture layer, (c) a negativeelectrode comprising an active material mixture layer comprising anactive material comprising graphite capable of reacting to insert orextract lithium ions, and a second polymer absorbing and holding saidorganic electrolyte solution, and a negative current collectorsupporting said active material mixture layer, (d) a porous separatorhaving a third polymer for absorbing and holding said organicelectrolyte solution, and (e) a ligand selected from the groupconsisting of coronands, podanocoronands, cryptands, spherands, andmixtures thereof, wherein: said organic solvent comprises a mixedsolvent of either (1) a cyclic carbonate and a chain carbonate, or (2) acyclic carbonate, a chain carbonate, and an aliphatic carboxylic acidester, said mixed solvent comprises more than 80% of the total volume ofsaid organic solvent, said supporting salt comprises a substancecomprising at least one anion selected from the group consisting ofperchlorate, tetrafluoro borate, hexafluoro phosphate, trifluoromethanesulfonate, and trifluoromethane imide sulfonate, said ligand has astronger coordination selecting capacity than said organic solvent orsaid anion for lithium ions, said ligand has a cyclic structure having apore, said pore has a diameter of about 1.7 angstroms or more, theconcentration of said ligand in the battery is in a range of 110micromole to 1 millimole per 1 Ah the capacity of said battery, and saidligand is oriented at the interface of said positive electrode and saidelectrolyte solution, and at the interface of said negative electrodeand said electrolyte solution.
 15. The lithium secondary battery ofclaim 14, wherein said ligand is at least one ligand selected from thegroup consisting of coronands and cryptands.
 16. The lithium secondarybattery of claim 14, wherein said ligand is selected from the groupconsisting of 15-crown-5-ether, 18-crown-6-ether, and 2,2,1-cryptand.